We propose to investigate the binding of antibodies to platelets in patients with idiopathic thrombocytopenic purpura, thrombocytopenia with high platelet-bound IgG, drug-induced thrombocytopenia, and alloimmune thrombocytopenia due to HLA and other antigens. We propose to propose to identify the glycoproteins within the platelet structure with which these antibodies interact by radioimmunoprecipitation, crossed immuno-electrophoresis, and electrophoresis with "Western" blot. In order to classify more accurately the proteins with which these antibodies interact, a panel of monoclonal antibodies which are made in mouse hybridomas and which identify various proteins of the platelet will be made. The proteins with which these antibodies react will be compared to results with clinically significant antibodies. Proof that the monoclonal antibody and the clinically significant antibody react with the same protein will be obtained by inhibition studies and by antigen removal studies. The identity of the protein will be determined by electrophoretic mobility on functional tests. IgG may also bind by Fc receptors. When IgG is bound by the Fab portion of the molecule, the Fc receptor is occupied. We propose to determine whether this is by the Fc portion of the antibody molecule. We propose to investigate quantitatively the relationships between the binding of the Fc receptor and the generation of the release reaction of the platelet.